Estimating by rounding means finding an answer that is close to the exact answer, but easier and faster to calculate. We round numbers to the nearest place value (like 10, 100, or 1,000) before subtracting, so the subtraction is simpler.
Estimation does not give the exact answer—it gives a close answer that helps us check reasonableness or solve problems quickly.
To round a number, look at the digit to the right of the place value you are rounding to. If it is 5 or more, round up. If it is less than 5, round down.
Always check which place value you are rounding to: tens, hundreds, thousands, or higher.
To estimate a subtraction problem, first round both numbers to the same place value. Then subtract the rounded numbers to get an estimated difference.
Rounding to different place values (10, 100, 1,000, 10,000, 100,000) will give different estimates. Choose the place value that makes sense for the situation.
Here are examples of estimating by rounding to different place values.
Notice how the estimate changes depending on the place value you round to. Rounding to a larger place value makes the estimate less exact but faster.
Estimation is useful when you want a quick answer, to check if an exact answer makes sense, or when exact details are not required.
Estimation is about being close enough, not exact. Use it for speed, reasonableness, and problem-solving.